The present invention relates to testing of metallic objects for the presence or absence of defects, and more particularly to improvements in a method and apparatus for so-called stray field or stray flux magnetic testing of elongated ferromagnetic parts.
Stray field magnetic testing involves the placing of a part to be tested into a magnetic field whereby the part develops a stray field in the region of each defect which warrants detection. The stray field attracts a pulverulent magnetic material which is sprayed or otherwise delivered into contact with the part to be tested whereby the material which adheres to the part pinpoints the location of the defect. Such mode of testing ferromagnetic parts is often resorted to in steel making and steel processing plants in order to detect defects in the surface of a semifinished product prior to incurring the expense of further processing. The defects are remedied or the defective parts are segregated from satisfactory parts.
Heretofore known stray field magnetic testing methods and apparatus satisfactory as far as the detection of defects is concerned. However, conventional methods and apparatus often necessitate two or more passes of the parts to be tested through the testing station, or the transport of the parts to be tested through several testing stations, each of which prolongs the testing operation and contributes to its cost. This is due to the fact that, when tested in accordance with heretofore known methods and/or in heretofore known apparatus, all sides of the parts to be tested cannot be simultaneously contacted with a mass of pulverulent magnetic material (e.g., ferritic powder) or all sides cannot be contacted with such material to the same extent.
German Offenlegungsschrift No. 21 16 827 discloses an apparatus for destruction-free testing of ferromagnetic parts in a continuous operation. The apparatus includes means for spraying a magnetic powder emulsion onto the running semifinished part. Such mode of testing creates many problems during actual testing as well as thereafter if all traces of the applied emulsion are to be removed from the surface of the tested commodity. Moreover, the apparatus which is disclosed in the aforementioned Offenlegungsschrift is not suited for the testing of ferromagnetic parts at elevated temperatures, and such apparatus must be provided with several testing stations if the entire peripheral surface of a moving part is to be tested during a single pass through the apparatus. This contributes significantly to the initial and maintenance cost of the apparatus. Reference may also be had to U.S. Pat. No. 3,763,423 granted Oct. 2, 1973 to Forster which discloses an apparatus with two testing stations and means for spraying a magnetic powder emulsion at several locations against selected portions of the advancing part. The apparatus of Forster advances the parts longitudinally while holding the advancing parts against rotation in order to ensure that each discrete spray of magnetic powder emulsion will impinge upon a selected strip-shaped portion of the peripheral surface of the ferromagnetic part.
Another mode of testing a continuously advancing ferromagnetic part in accordance with the stray field magnetic technique is disclosed in European patent application Ser. No. 0 090 190. The apparatus of this application employs dry magnetic powder and is similar to that disclosed in U.S. Pat. No. 3,614,604 granted Oct. 19, 1971 to Reinshagen. Reinshagen proposes to apply to the running ferromagnetic part a spray which is a mixture of dry magnetic powder and a gaseous carrier medium whereby the powder adheres to the locations which exhibit defects, and such locations are thereupon detected by a monitoring device which, in turn, controls a source of liquid marking material which is sprayed onto and around the accumulations of powder adhering to the ferromagnetic part so that, when the powder is removed, the marking material surrounds the thus exposed defective portion of the tested part. The patentee further proposes to heat the tested part and to thus bond the powder to the periphery of the part in lieu of the application of a liquid marking material. Each of these proposals is unsatisfactory because the apparatus is complex and all portions of the peripheral surface of a tested part are not moved to optimum positions with reference to the poles of the electromagnet or electromagnets.